RESUMO
Mammalian orthoreovirus (MRV) outer capsid protein σ3 is a multifunctional protein containing a double-stranded RNA-binding domain, which facilitates viral entry and assembly. We reasoned that σ3 has an innate immune evasion function. Here, we show that σ3 protein localizes in the mitochondria and interacts with mitochondrial antiviral signaling protein (MAVS) to activate the intrinsic mitochondria-mediated apoptotic pathway. Consequently, σ3 protein promotes the degradation of MAVS through the intrinsic caspase-9/caspase-3 apoptotic pathway. Moreover, σ3 protein can also inhibit the expression of the components of the RNA-sensing retinoic acid-inducible gene (RIG)-like receptor (RLR) signaling pathway to block antiviral type I interferon responses. Mechanistically, σ3 inhibits RIG-I and melanoma differentiation-associated gene 5 expression is independent of its inhibitory effect on MAVS. Overall, we demonstrate that the MRV σ3 protein plays a vital role in negatively regulating the RLR signaling pathway to inhibit antiviral responses. This enables MRV to evade host defenses to facilitate its own replication providing a target for the development of effective antiviral drugs against MRV. IMPORTANCE: Mammalian orthoreovirus (MRV) is an important zoonotic pathogen, but the regulatory role of its viral proteins in retinoic acid-inducible gene-like receptor (RLR)-mediated antiviral responses is still poorly understood. Herein, we show that MRV σ3 protein co-localizes with mitochondrial antiviral signaling protein (MAVS) in the mitochondria and promotes the mitochondria-mediated intrinsic apoptotic pathway to cleave and consequently degrade MAVS. Furthermore, tryptophan at position 133 of σ3 protein plays a key role in the degradation of MAVS. Importantly, we show that MRV outer capsid protein σ3 is a key factor in antagonizing RLR-mediated antiviral responses, providing evidence to better unravel the infection and transmission mechanisms of MRV.
RESUMO
Mammalian reovirus (MRV) is a non-enveloped, gene segmented double-stranded RNA (dsRNA) virus. It is an important zoonotic pathogen that infects many mammals and vertebrates that act as natural hosts and causes respiratory and digestive tract diseases. Studies have reported that RIG-I and MDA5 in the innate immune cytoplasmic RNA-sensing RIG-like receptor (RLR) signaling pathway can recognize dsRNA from MRV and promote antiviral type I interferon (IFN) responses. However, the mechanism by which many MRV-encoded proteins evade the host innate immune response remains unclear. Here, we show that exogenous µ1 protein promoted the proliferation of MRV in vitro, while knockdown of MRV µ1 protein expression by shRNA could impair MRV proliferation. Specifically, µ1 protein inhibited MRV or poly(I:C)-induced IFN-ß expression, and attenuated RIG-I/MDA5-mediated signaling axis transduction during MRV infection. Importantly, we found that µ1 protein significantly decreased IFN-ß mRNA expression induced by MDA5, RIG-I, MAVS, TBK1, IRF3(5D), and degraded the protein expression of exogenous MDA5, RIG-I, MAVS, TBK1 and IRF3 via the proteasomal and lysosomal pathways. Additionally, we show that µ1 protein can physically interact with MDA5, RIG-I, MAVS, TBK1, and IRF3 and attenuate the RIG-I/MDA5-mediated signaling cascades by blocking the phosphorylation and nuclear translocation of IRF3. In conclusion, our findings reveal that MRV outer capsid protein µ1 is a key factor in antagonizing RLRs signaling cascades and provide new strategies for effective prevention and treatment of MRV infection.